THERMOCHRONOLOGY OF THE SOUTH CYCLADES SHEAR ZONE, IOS, GREECE - EFFECTS OF DUCTILE SHEAR IN THE ARGON PARTIAL RETENTION ZONE

Micas and potassium feldspars from the South Cyclades Shear Zone, Ios, Cyclades, Greece, yield varied and complex Ar-40/Ar-39 apparent age s pectra. A correlation exists between Ar-40/Ar-39 apparent ages and the relative timing of different episodes of recrystallization and grain growth, as indicated by fabric and microstructural analysis. The Ar-40 /Ar-39 apparent age spectra record the effects of variation in the deg ree of recrystallization and grain growth, and partial to complete res etting of argon systematics in potassium-bearing minerals during Hercy nian (M-0), Alpine (M-1) and late Oligocene-early Miocene (M-2) metamo rphism. Deformation was strongly partitioned within the shear zone and this led to localized recrystallization and heterogeneous resetting o f argon systematics within preexisting minerals. Modeling suggests the Oligo-Miocene thermal events were of insufficient magnitude and/or du ration to completely reset the isotopic systematics in these samples. Our data lead to the concept of the argon partial retention and resett ing zone (PRZ), defined as that portion of the crust where temperature s are insufficient to completely reset argon systematics within preexi sting potassium-bearing minerals. Within the PRZ, some radiogenic argo n in preexisting potassium-bearing minerals will be outgassed and only partially retained. Tectonic exhumation of the PRZ involves movement on crustal-scale ductile shear zones, accompanied by strongly partitio ned deformation and localized recrystallization. Recrystallization lea ds to resetting of argon systematics, and thus will result in heteroge neous Ar-40/Ar-39 age distributions within these ductile shear zones ( e.g., in the South Cyclades Shear Zone).